Silica gel carrying titanium oxide photocatalyst

ABSTRACT

The silica gel carrying a titanium oxide photocatalyst in a high concentration of the present invention is characterized in that the concentration of the titanium oxide contained in the pores near the surface of the silica gel is 7-70% by weight, said silica gel having an average pore diameter of 6-100 nm, provided that the concentration gradient is provided such that the amount of the titanium oxide contained in the pores near the surface of the silica gel is 1.5 times or more than the amount of the titanium oxide contained in the pores near the central part of the silica gel.

FIELD OF THE INVENTION

This invention relates to silica gel carrying a titanium oxidephotocatalyst in a high concentration that is capable of deodorizingautomobiles, rooms, and stockbreeding pens or detoxifying toxicpollutants, of removing NOx in the atmosphere, decolorizing waste watercaused by dyeing, preventing the emergence of algae in water tanks, andsterilizing water and air, in which the titanium oxide is contained in ahigh concentration in the pores of the silica gel, which has anadsorption performance, and relates to a method for preparation thereof.

BACKGROUND OF THE INVENTION

It has been well known that for a photocatalytic reaction of asemiconductor, when powders of a semiconductor are dispersed in a watersolution to which a light having the same amount or more of energy asthat of the band gap of the semiconductor (light having a wavelength of400 nm or less) is made to irradiate, electrons and electron holesgenerated by optical pumping are transferred onto the surface ofsemiconductor particles and act on an ionic species and a molecularspecies in the water solution to cause various reactions, such as thedecomposition of water. Titanium oxide is exemplified as arepresentative photocatalyst.

By irradiating a semiconductor by light such as sunlight, or the lightof a fluorescent tube, incandescent lamp, black light, lamp,metal-halide lamp, or a cold cathode fluorescent tube, the decompositionand removal of environmental pollutants can be conducted, such as thedecomposition and removal of smelly and toxic substances in the air, thetreatment of waste water, a water-purifying treatment, or thesterilization of microorganisms in water.

Due to the difficulty of handling fine powders for those uses, suggestedin Japanese Patent Early-publication No. 6-65012 is a titanium oxidefilm photocatalyst that is excellent in water-resistant andheat-resistant properties and in durability. It is obtained by producinga sol of titanium oxide from an alkoxide of titanium, coating the sol ona glass substrate by dip coating, drying and calcining the sol-coatedglass substrate to form a transparent titanium oxide film photocatalyst,or by further coating a metal on the photocatalyst byphoto-electrodeposition, etc. However, it has in its use big problems,such as that this catalyst can be applied only to an inorganicsubstance, the shape of which is limited, such as glass, ceramics, etc.,and also that since a decomposition reaction by a photocatalyst iscaused only on the surface of the photocatalyst, to continuouslydecompose and remove environmental pollutants a very large area must beprovided for the photocatalyst.

DISCLOSURE OF THE INVENTION

Considering these problems, the present invention has been achieved.This invention provides silica gel carrying a photocatalyst in a highconcentration and a method for preparation thereof. The silica gel ofthis invention has an improved performance in decomposing environmentalpollutants such as smelly or toxic substances in the air, or organicsolvents, agricultural chemicals, etc., contained in water, by providinga concentration gradient of titanium oxide such that the concentrationnear the surface of the silica gel is high and the concentration of thecentral part is low, said silica gel also having excellent properties ofsafety, economic efficiency, stability, and water-resistance (even if itis put into water it does not break apart).

This invention provides silica gel carrying a titanium oxidephotocatalyst in a high concentration characterized in that, to achievethe above objects, the amount of the titanium oxide contained in thepores near the surface of the silica gel is 7-70% by weight, providedthat a concentration gradient is provided such that the amount is 1.5times or more than the amount of the titanium oxide contained in thepores near the center of the silica gel, said silica gel having anaverage pore diameter of 6-100 nm. This invention also provides a methodfor the preparation of silica gel carrying a titanium oxidephotocatalyst in a high concentration characterized by mixing silica gelwith a titanium-containing solution in the same volume as or less thanthe total pore volume of the silica gel, to have the silica gel containthe titanium-containing solution, followed by heating and calcining,said silica gel having an average pore diameter of 6-100 nm.

The silica gels used in the present invention are amorphous silicondioxides having an average pore diameter of 6-100 nm, said silica gelnot breaking apart even if it is immersed in water or an organicsolvent. They can be obtained by a known method such as is described inJapanese Patent Publication No. 7-64543, etc. In this method a silicahydrogel obtained by neutralizing an alkaline silicate water-solution isdried at 100-1,000° C. by superheated steam to result in a silicaxerogel, to adjust the sizes of the colloidal particles that constitutethe silica gel, or by obtaining Q-6, Q-10, Q-15, Q-30, or Q-50, whichare commercially available as series of CARiACT Q (produced byFuji-Silysia Chemical Ltd.). Those silica gels greatly differ fromsilica gel generally used as a desiccant for packaging (an A-type orB-type standardized product under JIS Z 0701, desiccant for packaging).Since they have good mechanical strength, adsorption performance, andthe property of not breaking apart even if they are immersed in water oran organic solvent, they are also used for chromatography or catalyticsupports. They are applicable in various forms, from fine powders togranules. The desiccant as described above that is used for packaginghas a critical defect in that when it is immersed in water or an organicsolvent it crumbles. Thus it cannot be used for the present invention.

In the present invention, when the average pore diameter of the silicagel used is less than 6 nm the titanium-containing solution clogs thepores near the surface of the silica gel and thus the solution does notsufficiently impregnate the internal parts. Also, when silica gel has anaverage pore diameter of more than 100 nm, it is difficult to produce itand it is very expensive, and thus it is undesirable.

The crystalline structure of the titanium oxide to be contained in thepores of the silica gel of the present invention is desirably anatase,which has a high photocatalytic activity.

The silica gel carrying a titanium oxide photocatalyst in a highconcentration of the present invention is one in which the amount oftitanium oxide contained in pores near the surface of silica gel is7-70% by weight, and a concentration gradient is provided such that theamount is 1.5 times or more than the amount of the titanium oxidecontained in the pores near the center of the silica gel. Thephotocatalytic reaction of the titanium oxide occurs with an ultavioletlight having a wavelength of 400 nm or less. For an ultraviolet lighthaving a wavelength of 380 nm, about 60% of the light is absorbed in atitanium oxide film having a thickness of 1 μm, and the titanium oxidethat is involved with the photocatalytic reaction is the only onepresent near the surface of the silica gel. Since a titanium-containingsolution is very expensive, the silica gel carrying a titanium oxidephotocatalyst in a high concentration of the present invention iseconomically very excellent. The silica gel of this invention containstitanium oxide in a high concentration in the pores near its surface,while the titanium oxide is contained in a lower concentration or noneat all in the pores of the inner part, where ultraviolet light does notreach and thus no photocatalytic reaction occurs. The titanium oxidecontent contained in the pores near the surface of the silica gel can bedetermined by EPMA, which can measure to a depth of 1-2 μm from thesurface of the silica gel. The titanium oxide content contained in thepores near the center of the silica gel can be determined by EPMA bymeasuring the cross section obtained by cracking the silica gel.

The titanium oxide-containing solution used in the present invention maybe, but is not limited to, organic titanium-containing solutions such astetraisopropyl titanate, tetrabutyl titanate, butyl titanate dimer,tetrakis(2-ethylhexyloxy)titanium, tetrastearyl titanate,triethanolamine titanate, alkoxides of titanium such asdiisopropoxy-bis(acetylacetonato)titanium,dibutoxy-bis(triethanolaminato)titanium, titanium ethylacetoacetate,titanium isopropoxyoctylene glycolate, titanium lactate, andtitanate-based coupling agents such as isopropyltriisostearoyl titanate,isopropyltridodecylbenzenesulfonyl titanate,isopropyltris(dioctylpyrophosphate)titanate,tetraisopropylbis(dioctylphosphite)titanate,tetraoctylbis(ditridecylphosphite)titanatetetra(2,2-dialyloxymethyl-1-butyl)bis(ditridecyl)phosphite titanate,bis(dioctylpyrophosphate)oxyacetate titanate,bis(dioctylpyrophosphate)ethylene titanate,isopropyltri(dioctylphophate)titanate, isopropyltricumylphenyl titanate,and isopropyltri(N-amidoethyl-aminoethyl)titanate, and inorganictitanium-containing solutions such as titanium sulfate, titaniumchloride, and titanium bromide.

Also, the above titanium-containing solution can be used alone or as amixture of two or more of them, without limitation. It may be dilutedwith a solvent that is compatible therewith to adjust its concentration.As a diluent, any one that is compatible with the titanium-containingsolution, such as ethanol, 1-propanol, 2-propanol, n-hexane, benzene,toluene, xylene, trichlene, propylene dichloride, and water can be usedalone or as a mixture of two or more of them, without limitation.

The silica gel carrying a titanium oxide photocatalyst in a highconcentration of the present invention may be obtained by adding to,e.g., a cylindrical vessel provided with a cap, the silica gel having anaverage pore diameter in a range of 6-100 nm as stated above and atitanium-containing solution in the same volume as or less than thetotal pore volume of the silica gel, rotating, vibrating, or shaking thevessel to cause the silica gel to contain a titanium-containingsolution, and heating and calcining the resulting silica gel. Usuallywhen this procedure is conducted once, a sufficient amount of titaniumoxide being carried can be obtained. However, when the same procedure isconducted plural times, the amount being carried can be furtherincreased.

When a titanium-containing solution is made to be contained in thesilica gel, it is not preferable to use a conventional method such asthe impregnating method, which has been generally used. This is becausea titanium-containing solution in an amount greater than the total porevolume that the silica gel has is used, and thereby an excesstitanium-containing solution covers the surface of silica gel, which isthen calcined by heating to form a titanium oxide film that has a verysmall specific surface and that tends to peel. Even if atitanium-containing solution is used in an amount greater than the totalpore volume of the silica gel, if an excess amount of thetitanium-containing solution that covers the surface of the silica gelis removed by washing it with a diluting liquid etc., a titanium oxidefilm on the surface of the silica gel is not formed when heating andcalcining the silica gel, and thus the above problem is overcome.However, this method is not preferable because a large amount of wasteliquid caused by washing it results, and a very expensivetitanium-containing solution is wasted.

To obtain a titanium oxide having a high performance as a photocatalyst,the crystalline form of which is anatase, the calcination by heating thesilica gel containing a titanium-containing solution of the presentinvention is conducted by heating it by gradually raising thetemperature of it from room temperature, maintaining the finaltemperature of 400-700° C. for a certain period, followed by cooling itto room temperature. If the calcination temperature is lower than 400°C., or higher than 700° C., a titanium oxide results in which the rutileor amorphous form is mixed. Thus such a temperature is not preferable.

As the furnace to be used for calcination, a gas furnace that providesoxygen sufficient for the calcination is desirable. However, an electricfurnace in which insufficient oxygen is provided can also be used forthe calcination, without causing a problem if more oxygen is added. Whenthe procedure of carrying titanium oxide and calcination is conductedtwice, more titanium oxide can be carried.

When a titanium-containing solution in a volume less than the total porevolume of the silica gel is made to be contained in the silica gel, thesolution slowly infiltrates from the surface of the silica gel into thecenter of it, to form hollow spheres remaining as cavities in the centerof the silica gel. Heating and calcining the thus-obtained silica gelgives the silica gel carrying a titanium oxide photocatalyst in a highconcentration of the present invention, with only the surface parts ofit containing titanium oxide. Also, even if a titanium oxide solution ismade to be contained in the same volume as the total pore volume of thesilica gel, if the concentration of a diluting liquid contained in thetitanium-containing solution is 5-75% by weight and if the dilutingliquid has a low molecular weight, due to the filtering function of thepores of the silica gel the silica gel that is provided with a gradientin the concentration of titanium oxide in it is obtained such that thenearer the surface part of the silica gel, the higher the concentrationis, and the nearer the central part of it, the lower the concentrationis. The thus-obtained silica gel is heated and calcined to result insilica gel of the present invention carrying a titanium oxidephotocatalyst in a high concentration provided with a gradient such thatthe nearer the surface part of the silica gel, the higher theconcentration of titanium oxide is, and the nearer the central part ofit, the lower the concentration of titanium oxide is.

When a titanium-containing solution that is readily hydrolyzable isused, the degree of the hydrolyzation of the solution is higher forsilica gel having a smaller average pore diameter. However, titaniummore promptly reacts with the silanol groups present on the surface ofthe silica gel. Thus the solution is contained on the surface part in ahigher concentration. The thus-obtained silica gel of the presentinvention is calcined to result in silica gel carrying a titanium oxidephotocatalyst in a high concentration of the present invention, saidsilica gel being provided with a gradient in the concentration oftitanium oxide such that the nearer the surface part of the silica gel,the higher the concentration is, and the nearer the central part, thelower the concentration is.

Although the period for mixing the silica gel and a titaniumoxide-containing solution varies based on the kind of titaniumoxide-containing solution to be used, it is one minute or more. It ispreferable that it be 30-60 minutes, because titanium can be containedin the silica gel that is provided with a more uniform concentrationgradient.

Also, when by a known method at least one kind of substance selectedfrom the transition elements of atomic numbers 21 (Sc) to 29 (Cu), 39(Y) to 47 (Ag), 57 (La) to 79 (Au), and 89 (Ac) to 103 (Lr), and anoxide thereof is made to co-exist with the titanium oxide photocatalystin the pores of the silica gel carrying the photocatalyst of the presentinvention, the photocatalyst activity may increase. Although the reasontherefor is unclear, a reaction is known as Fenton's reaction, and issuch that when light is applied to titanium oxide, the resultinghydrogen peroxide, which is one kind of active oxygen, is reacted with adivalent. ferrous ion to produce a hydroxide radical. Thus it isbelieved that a mechanism similar to this operates with the transitionelements other than iron.

The silica gel of the present invention, carrying a titanium oxidephotocatalyst in a high concentration, can adsorb odors and toxicsubstances in the air, and organic solvents and agricultural chemicalsin water in a larger amount, by containing titanium oxide in the poresof the silica gel, said pores having a larger specific surface, thesilica gel being provided with a gradient in the concentration oftitanium oxide. By irradiating on it light such as sunlight, or thelight of a fluorescent tube, an incandescent lamp, black light, anultraviolet lamp, mercury-arc lamp, xenon lamp, tungsten-halogen lamp,metal-halide lamp, or a cold cathode fluorescent tube, it can decomposetoxic substances, etc. more efficiently than can the silica gel of anyconventional invention in which a titanium oxide film photocatalyst isfixed only on the surface of silica gel.

Also, since in the present invention silica gel carrying a titaniumoxide photocatalyst not only in a particle form, but also in a powderform, is obtained, on the surface of which little titanium oxide ispresent, it can be incorporated in plastics, paints and varnishes,paper, etc., which are decomposable with titanium oxide, to make themhave functions such as an antibacterial action, and functions for thedeodorization and decomposition of dirt.

The present invention will be explained in detail by the followingExamples.

EXAMPLE 1 An Example in Which an Organic Titanium-containing SolutionThat is Hardly Hydrolyzable was Used

250 g of silica gel (an average pore diameter of 10 nm; a pore volume of1.0 ml; a specific surface of 300 m²/g) remaining on an 8 mesh-sieveafter sieving, i.e., having a particle diameter of 2.38 mm or more, anddried at 200° C., and 150 g of diisopropoxy-bis(acetylacetonato)titanium(the content in terms of titanium oxide was 16.5% by weight), saidamount being 60% of the total pore volume of the silica gel, was placedin a polyethylene vessel. The vessel was promptly capped and was placedon a pot mill pedestal. The pedestal was rotated at 20 rpm for 1 hour.After that the obtained silica gel was heated by gradually raising thetemperature from room temperature to 600° C. using an electric furnacewhile sometimes the furnace cap was opened and oxygen was provided, andthe temperature was maintained at 600° C. for 1 hour. Then thethus-obtained silica gel was naturally cooled to room temperature, toresult in the silica gel of the present invention carrying a titaniumoxide photocatalyst in a high concentration.

The thus-obtained silica gel carrying a titanium oxide photocatalyst ina high concentration was examined by X-ray diffraction. As a result, thecrystalline structure of the titanium oxide was found to be 100%anatase. When the titanium oxide concentration on the surface of thesilica gel was determined by EPMA, it was found to be 39% by weight.When the titanium oxide concentration on the central part of the crosssection of the silica gel was determined, it was found to be about 0% byweight. Also, the content of titanium oxide determined from a measuredvalue of the true specific gravity of the obtained silica gel carrying atitanium oxide photocatalyst in a high concentration was 11.1% by weight(based on the dried weight at 200° C.).

A test for decoloring colored water created by dyeing was conductedusing this silica gel carrying a titanium oxide photocatalyst in a highconcentration. First, 1.25 g of the silica gel carrying a titanium oxidephotocatalyst in a high concentration and 1.25 g of only silica gel asthe control were each placed in a separate quartz cell for aspectrophotometer (external size: 12.5 mm square ×45 mm height). To eachof them, 4 ml of previously prepared colored water created by dyeing(water that contained methylene blue in an amount of 100 ppm) was added,and then light by a black light (15 W) was made to irradiate from aplace that was 10 mm away from the cell, and the result was that thecell in which the silica gel carrying a titanium oxide photocatalyst ina high concentration was placed turned almost transparent after 60minutes, while the cell for the control still remained its originalblue. From this result it was recognized that the silica gel of thepresent invention carrying a titanium oxide photocatalyst in a highconcentration had a significant performance on decomposing coloredwater.

EXAMPLE 2 An Example in Which an Organic Titanium-containing SolutionThat is Readily Hydrolyzable was Used

250 g of silica gel (an average pore diameter of 10 nm; a pore volume of1.0 ml; a specific surface of 300 m²/g) remaining on an 8 mesh-sieveafter sieving, i.e., having a particle diameter of 2.38 mm or more, anddried at 200° C. , and 250 g of titanium tetraisopropoxide (the contentin terms of titanium oxide was 28.2% by weight), said amount being 100%of the total pore volume of the silica gel, were placed in apolyethylene vessel. The vessel was promptly capped and was placed on apot mill pedestal. Then the pedestal was rotated at 20 rpm for 1 hour.After that the obtained silica gel was heated by gradually raising thetemperature from room temperature to 600° C. using an electric furnacewhile sometimes the furnace cap was opened and oxygen was provided, andthe temperature was maintained at 600° C. for 1 hour. Then thethus-obtained silica gel was naturally cooled to room temperature, toresult in silica gel of the present invention carrying a titanium oxidephotocatalyst in a high concentration.

The thus-obtained silica gel carrying a titanium oxide photocatalyst ina high concentration was examined by X-ray diffraction. As a result, thecrystalline structure of the titanium oxide was found to be 100%anatase. When the titanium oxide concentration on the surface of thesilica gel was determined by EPMA, it was found to be 45% by weight.When the titanium oxide concentration on the central part of the crosssection of the silica gel was determined, it was found to be 18% byweight. Also, the content of titanium oxide determined from a measuredvalue of the true specific gravity of the obtained silica gel carrying atitanium oxide photocatalyst in a high concentration was 25.3% by weight(based on the dried weight at 200° C.).

A test for decoloring colored water caused by dyeing was conducted usingthis silica gel carrying a titanium oxide photocatalyst in a highconcentration. First, 1.25 g of the silica gel carrying a titanium oxidephotocatalyst in a high concentration and 1.25 g of only silica gel asthe control were each placed in a separate quartz cell for aspectrophotometer (external size: 12.5 mm square×45 mm height). To bothof them, 4 ml of previously prepared colored water created by dyeing(water that contains methylene blue in an amount of 100 ppm) was added.Then light by a black light (15 W) was made to irradiate from a placethat was 10 mm away from the cell, and the result was that the cell inwhich the silica gel carrying a titanium oxide photocatalyst in a highconcentration was placed turned almost transparent after 60 minutes,while the control cell still remained its original blue. From thisresult it was recognized that the silica gel carrying a titanium oxidephotocatalyst in a high concentration of the present invention had asignificant performance in decomposing the colored water.

EXAMPLE 3 An Example in Which an Inorganic Titanium-containing Solutionwas Used

250 g of silica gel (an average pore diameter of 10 nm; a pore volume of1.0 ml; a specific surface of 300 m²/g) remaining on an 8 mesh-sieveafter sieving, i.e., having a particle diameter of 2.38 mm or more, anddried at 200° C., and 180 g of a 20% water solution of titaniumtrichloride (the content in terms of the titanium oxide was 10.3% byweight), said amount being 60% of the total pore volume of the silicagel, were placed in a polyethylene vessel. The vessel was promptlycapped and was placed on a pot mill pedestal. Then the pedestal wasrotated at 20 rpm for 1 hour. After that the obtained silica gel washeated by gradually raising the temperature from room temperature to600° C., using an electric furnace, while sometimes the furnace cap wasopened and oxygen was provided, and the temperature was maintained at600° C. for 1 hour. Then the thus-obtained silica gel was naturallycooled to room temperature to result in the silica gel carrying atitanium oxide photocatalyst in a high concentration of the presentinvention.

The thus-obtained silica gel carrying a titanium oxide photocatalyst ina high concentration was examined by X-ray diffraction. As a result, thecrystalline structure of the titanium oxide was found to be 100%anatase. When the titanium oxide concentration on the surface of thesilica gel was determined by EPMA, it was found to be 35% by weight.When the titanium oxide concentration on the central part of the crosssection of the silica gel was determined, it was found to be about 0% byweight. Also, the content of titanium oxide determined from a measuredvalue of the true specific gravity of the obtained silica gel carrying atitanium oxide photocatalyst in a high concentration was 8.3% by weight(based on the dried weight at 200° C.).

A test for decoloring colored water created by dyeing was conductedusing this silica gel carrying a titanium oxide photocatalyst in a highconcentration. First, 1.25 g of the silica gel carrying a titanium oxidephotocatalyst in a high concentration and 1.25 g of only silica gel asthe control were each placed in a separate quartz cell for aspectrophotometer (external size: 12.5 mm square×45 mm height). To eachof them, 4 ml of previously prepared colored water created by dyeing(water that contains methylene blue in an amount of 100 ppm) was added.Then light by a black light (15 W) was made to irradiate from a placethat was 10 mm away from the cell, and the result was that in the cellin which the silica gel carrying a titanium oxide photocatalyst in ahigh concentration was placed turned almost transparent after 60minutes, while the cell for the control still remained its originalblue. From this result it was recognized that the silica gel carrying atitanium oxide photocatalyst in a high concentration of the presentinvention had a significant performance in decomposing the coloredwater.

Comparative Example 1 An Example in Which the Same Kind of Silica Gel asin the Above Examples was used Except That it had a Smaller Average SoreDiameter

250 g of silica gel (an average pore diameter of 3 nm; a pore volume of0.3 ml; a specific surface of 550 m²/g) remaining on an 8 mesh-sieveafter sieving, i.e., having a particle diameter of 2.38 mm or more, anddried at 200° C. , and 75 g of diisopropoxy bis(acetylacetonato)titanium(the content in terms of titanium oxide was 16.5% by weight ), saidamount being 100% of the total pore volume of the silica gel, wereplaced in a polyethylene vessel. The vessel was promptly capped and wasplaced on a pot mill pedestal. The pedestal was rotated at 20 rpm for 1hour. After that the obtained silica gel was heated by gradually raisingthe temperature from room temperature to 600° C. using an electricfurnace while sometimes the furnace cap was opened and oxygen wasprovided, and the temperature was maintained at 600° C. for 1 hour. Thenthe thus-obtained silica gel was naturally cooled to room temperature toresult in the silica gel carrying a photocatalyst.

The thus-obtained silica gel carrying a photocatalyst was examined byX-ray diffraction. However, no crystalline structure of the titaniumoxide could be determined because of the small amount of titanium oxidecarried. When the titanium oxide concentration on the surface of thesilica gel was determined by EPMA, it was found to be 1.2% by weight.When the titanium oxide concentration on the central part of the crosssection of the silica gel was determined, it was found to be about 0% byweight. Also, the content of titanium oxide determined from a measuredvalue of the true specific gravity of the obtained silica gel carrying aphotocatalyst was 0.3% by weight (based on the dried weight at 200° C.).

A test for decoloring colored water created by dyeing was conductedusing this silica gel carrying a photocatalyst. First, 1.25 g of thesilica gel carrying a photocatalyst and 1.25 g of only silica gel as thecontrol were each placed in a separate quartz cell for aspectrophotometer (external size: 12.5 mm square×45 mm height). To eachof them, 4 ml of previously prepared colored water created by dyeing(water that contains methylene blue in an amount of 100 ppm) was added,and then light by a black light (15 W) was made to irradiate from aplace that was 10 mm away from the cell, and the result was that thecell in which the silica gel carrying a photocatalyst was placedremained blue 360 minutes later. From this result it was recognized thatthe silica gel carrying a photocatalyst had little performance indecomposing colored water.

Comparative Example 2 An Example in Which a Different Kind of Silica Gel(An A-Type Standardized Product Under JIS Z 0701, Desiccant Used forPackaging) was Used

In the same manner as in Example 2, 250 g of A-type silica gel (anaverage pore diameter of 2.4 nm; a pore volume of 0.46 ml; a specificsurface of 700 m²/g) remaining on an 8 mesh-sieve after sieving, i.e.,having a particle diameter of 2.38 mm or more, and dried at 200° C. ,and 115 g of diisopropoxybis(acetylacetonato)titanium (the content interms of titanium oxide was 16.5% by weight ), said amount being 100% ofthe total pore volume, were placed in a polyethylene vessel. The vesselwas promptly capped and was placed on a pot mill pedestal. When thepedestal was rotated at 20 rpm for 1 hour, it was found that the silicagel had broken into pieces. It was recognized as being incapable ofbeing used.

EXAMPLE 4 An Example in Which a Titanium Coupling-agent was Used

250 g of silica gel (an average pore diameter of 10 nm; a pore volume of1.0 ml; a specific surface of 300 m²/g) remaining on an 8 mesh-sieveafter sieving, i.e., having a particle diameter of 2.38 mm or more, anddried at 200° C., and 150 g of isopropyltriisostearoyl titanate (thecontent in terms of titanium oxide was 8.2% by weight), said amountbeing 60% of the total pore volume of the silica gel, were placed in apolyethylene vessel. The vessel was promptly capped and was placed on apot mill pedestal. Then the pedestal was rotated at 20 rpm for 1 hour.After that the obtained silica gel was heated by gradually raising thetemperature from room temperature to 600° C. using an electric furnacewhile sometimes the furnace cap was opened and oxygen was provided, andthe temperature was maintained at 600° C. for 1 hour. Then thethus-obtained silica gel was naturally cooled to room temperature toresult in the silica gel carrying a titanium oxide photocatalyst in ahigh concentration of the present invention.

The thus-obtained silica gel carrying a titanium oxide photocatalyst ina high concentration was examined by X-ray diffraction. As a result, thecrystalline structure of the titanium oxide was found to be 100%anatase. When the titanium oxide concentration on the surface of thesilica gel was determined by EPMA it was found to be 32% by weight. Whenthe titanium oxide concentration on the central part of the crosssection of the silica gel was determined, it was found to be about 0% byweight. Also, the content of titanium oxide determined from a measuredvalue of the true specific gravity of the obtained silica gel carrying atitanium oxide photocatalyst in a high concentration was 4.7% by weight(based on the dried weight at 200° C.).

A test for decoloring colored water created by dyeing was conductedusing this silica gel carrying a titanium oxide photocatalyst in a highconcentration. First, 1.25 g of the silica gel carrying a titanium oxidephotocatalyst in a high concentration and 1.25 g of only silica gel asthe control were each placed in a separate quartz cell for aspectrophotometer (external size: 12.5 mm square×45 mm height). To eachof them, 4 ml of previously prepared colored water created by dyeing(water that contains methylene blue in an amount of 100 ppm) was added.Then light by a black light (15 W) was made to irradiate from a placethat was 10 mm away from the cell, and the result was that the cell inwhich the silica gel carrying a titanium oxide photocatalyst in a highconcentration was placed turned almost transparent 60 minutes later,while resulting in the cell for the control still remaining its originalblue. From this result it was recognized that the silica gel carrying atitanium oxide photocatalyst in a high concentration of the presentinvention had a significant performance in decomposing colored water.

EXAMPLE 5 An Example in Which Silica Gel in a Powdery Form was Used

250 g of silica gel (an average pore diameter of 6 nm; a pore volume of1.1 ml; a specific surface of 450 m²/g) having an average particlediameter of 40 μm and dried at 200° C. and 165 g ofdiisopropoxy-bis(acetylacetanato)titanium (the content in terms oftitanium oxide was 16.5% by weight), said amount being 60% of the totalpore volume of the silica gel, were placed in a polyethylene vessel. Thevessel was promptly capped and was placed on a pot mill pedestal. Thenthe pedestal was rotated at 20 rpm for 1 hour. After that the obtainedsilica gel was heated by gradually raising the temperature from roomtemperature to 600° C. using an electric furnace while sometimes thefurnace cap was opened and oxygen was provided, and the temperature wasmaintained at 600° C. for 1 hour. Then the thus-obtained silica gel wasnaturally cooled to room temperature to result in the silica gelcarrying a photocatalyst in a high concentration of the presentinvention.

The thus-obtained silica gel carrying a photocatalyst in a highconcentration was examined by X-ray diffraction. As a result, thecrystalline structure of the titanium oxide was found to be 100%anatase. When the titanium oxide concentration on the surface of thesilica gel was determined by EPMA, it was found to be 41% by weight.When the titanium oxide concentration on the central part of the crosssection of the silica gel was determined, it was found to be about 0% byweight. Also, the content of titanium oxide determined from a measuredvalue of the true specific gravity of the obtained silica gel carrying aphotocatalyst in a high concentration was 12.3% by weight (based on thedried weight at 200° C.).

A test for decoloring colored water created by dyeing was conductedusing this silica gel carrying a photocatalyst in a high concentration.First, 1.25 g of the silica gel carrying a photocatalyst in a highconcentration and 1.25 g of only silica gel as the control were eachplaced in a separate quartz cell for a spectrophotometer (external size:12.5 mm square×45 mm height). To each of them, 4 ml of previouslyprepared colored water created by dyeing (water that contains methyleneblue in an amount of 100 ppm) was added. Then light by a black light (15W) was made to irradiate from a place that was 10 mm away from the cell.This resulted in the cell in which the silica gel carrying a titaniumoxide photocatalyst in a high concentration was placed turning almosttransparent 60 minutes later, while the cell for the control stillremained its original blue. From this result it was recognized that thesilica gel carrying a photocatalyst in a high concentration of thepresent invention had a significant performance in decomposing coloredwater.

Comparative Example 3 An Example in Which Silica Gel in a Powdery Formand the Conventional Impregnation Method Were Used

To a beaker in which 50 g of diisopropoxy-bis(acetylacetonato)titanium(the content in terms of titanium oxide was 16.5% by weight ) wasplaced, 10 g of silica gel (an average pore diameter of 6 nm; porevolume of 1.1 ml; specific surface of 450 m²/g) having an averageparticle diameter of 40μm and dried at 200° C. was added. After thethus-obtained mixture was gently stirred, it was allowed to stand for 30minutes. Since no solid-liquid separation can be conducted by using asieve, it was done by centrifugal separation. However, the resultingsilica gel contained a solution. After that the silica gel was heated bygradually raising the temperature from room temperature to 600° C. usingan electric furnace while sometimes the furnace cap was opened andoxygen was provided, and the temperature was maintained at 600° C. for 1hour. Then the thus-obtained silica gel was naturally cooled to roomtemperature, to result in the silica gel carrying a photocatalyst.

Since the resulting silica gel was solidified, it was loosened manuallyfor observation by SEM. Ultrafine particles of titanium oxide that werenot originally present were observed in a large amount. It wasrecognized that the silica gel obtained did not differ from a meremixture of silica gel powders and ultrafine titanium oxide particles.

EXAMPLE 6

NOx were decomposed and removed by using the silica gel carrying aphotocatalyst in a high concentration that was obtained as in Example 1.First, in a sealed vessel having an internal volume of 40 l (a 20-Wblack light was built in), 1 kg of the obtained silica gel carrying aphotocatalyst in a high concentration was placed. After 10 ppm of NOxwas injected by a syringe in the sealed vessel, the light was turned on.After 15 minutes, the concentration of NOx contained in the air in thevessel was determined by a gas chromatograph and found to be 0 ppm. Thenafter the lowered amount of NOx was injected in the vessel by a syringe,the black light was again turned on. After 15 minutes, the concentrationof NOx contained in the air in the vessel was determined by a gaschromatograph and was found to be 0 ppm. The same determination wasrepeated 10 times. Every time, the concentration of NOx contained in theair in the vessel that was determined 15 minutes after the light wasturned on was 0 ppm. From this result it was recognized that the silicagel carrying a photocatalyst in a high concentration of the presentinvention had a significant effect on decomposing and removing NOx.

Next, a similar test was conducted in a sealed vessel using an activecarbon honeycomb (1 kg weight) instead of the silica gel carrying aphotocatalyst in a high concentration. As a result, it was found thatalthough the NOx concentration determined for the first time was about 0ppm, it gradually increased as the number of times increased. For thesixth time, it was about 10 ppm. This means that no decomposition orremoval effect was found at all.

EXAMPLE 7

A smelly substance was decomposed and removed by using the silica gelcarrying a titanium oxide in a high concentration that was obtained asin Example 2. First, in a sealed vessel having an internal volume of 40l (a 20-W black light was built in), 1 kg of the obtained silica gelcarrying a photocatalyst in a high concentration was placed. After 80ppm of trimethylamine was injected by a syringe in the sealed vessel,the light was turned on. After 20 minutes, the concentration of thetrimethylamine contained in the air in the vessel was determined by agas chromatograph and was found to be 0 ppm. Then after the loweredamount of trimethylamine was injected in the vessel by a syringe, theblack light was again turned on. After 20 minutes, the concentration oftrimethylamine contained in the air in the vessel was determined by agas chromatograph and found to be 0 ppm. The same determination wasrepeated 10 times. Every time, the concentration of the trimethylaminecontained in the air in the vessel that was determined 20 minutes afterthe light was turned on was 0 ppm. From this result it was recognizedthat the silica gel carrying a photocatalyst in a high concentration ofthe present invention had a significant effect on decomposing andremoving trimethylamine.

Next, a similar test was conducted in a sealed vessel using granularactive carbon (1 kg weight) instead of the silica gel carrying aphotocatalyst in a high concentration. As a result, it was found thatalthough the trimethylamine concentration determined for the first timewas 0 ppm, it gradually increased as the number of times increased. Forthe ninth time it was about 80 ppm, which means that no removal effectwas found at all.

EXAMPLE 8

An organic solvent was decomposed and removed by using the silica gelcarrying a titanium oxide in a high concentration that was obtained asin Example 4. First, in a test tube made of silica glass in which 15 mlof a water solution of 10 ppm of trichloroethylene was placed, 10 g ofthe silica gel carrying a photocatalyst in a high concentration wasplaced. After bubbling with oxygen, light of a 500-W high-pressuremercury-vapor lamp was made to irradiate. One hour after the light wasmade to irradiate on it, the concentration of the trichloroethylenecontained in the water solution was determined by a gas chromatographand found to be 0 ppm. Next, after a lowered amount of trichloroethylenewas added, light was applied. One hour after the light was made toirradiate, the concentration of trichloroethylene contained in the watersolution was determined by a gas chromatograph and found to be 0 ppm.The same determination was repeated 10 times. Every time theconcentration of the trichloroethylene contained in the water solutionwas determined one hour after the light was made to irradiate on it, itwas found to be 0 ppm. From this result it was recognized that thesilica gel carrying a photocatalyst in a high concentration of thepresent invention had a significant effect on decomposingtrichloroethylene.

Next, a similar test was conducted using a fibrous active carbonhoneycomb (1 kg weight) instead of the silica gel carrying aphotocatalyst in a high concentration. As a result, it was found thatalthough the trichlorethylene concentration determined for the firsttime was 0 ppm, it gradually increased as the number of times increased.For the tenth time, it was about 10 ppm, which means that nodecomposition effect was found at all.

EXAMPLE 9

A smelly substance was decomposed and removed by using the silica gelcarrying a photocatalyst in a high concentration that was obtained as inExample 5. First, a paste was thickly applied to the entire part of oneside of drawing paper. Before the paste was dried, the obtained silicagel carrying a photocatalyst in a high concentration was sprinkled onthe entire surface of the paste. Then the paste was dried to obtainpaper coated with silica gel carrying a photocatalyst. This paper waslaid in a sealed vessel having an internal volume of 40 l (a 20-W blacklight was built in). After injecting 80 ppm of trimethylamine in thesealed vessel by a syringe, the black light was turned on. After onehour, the concentration of trimethylamine contained in the air in thevessel was determined by a gas chromatograph and found to be 0 ppm. Thenafter the lowered amount of trimethylamine was injected in the vessel bya syringe, the black light was again turned on. After one hour, theconcentration of trimethylamine contained in the air in the vessel wasdetermined by a gas chromatograph and found to be 0 ppm. The samedetermination was repeated 10 times. Every time the concentration oftrimethylamine that was contained in the air in the vessel wasdetermined one hour after the light was turned on, it was 0 ppm. Fromthis result it was recognized that the silica gel carrying aphotocatalyst in a high concentration of the present invention had asignificant effect on decomposing and removing trimethylamine.

Next, a similar test was conducted using drawing paper coated withpowdery active carbon instead of the silica gel carrying a photocatalystin a high concentration. As a result, it was found that although thetrimethylamine concentration determined for the first time was 0 ppm, itgradually increased as the number of times increased. For the fifth timeit was about 80 ppm, which means that no decomposing effect was found atall.

EXAMPLE 10

A test was conducted to see if paint was decomposed when the silica gelcarrying a photocatalyst in a high concentration that was obtained inExample 5 was incorporated in the paint.

1% by weight of the silica gel carrying a photocatalyst in a highconcentration obtained as in Example 5 was added to a white acrylicemulsion paint (trade name: VINIDELUX; produced by Kansai Paint Co.,Ltd.). The thus-obtained paint was applied to a glass plate and dried atroom temperature for one week. After that the glass plate was placed 20cm under a black light (15W). The light was made to irradiate on thesilica gel for one week. No color change was observed at all. Incontrast, when a similar test was conducted for a commercially availablepowdery titanium oxide photocatalyst (AMT-600; produced by TaycaCorporation), the color of the paint was changed to yellow. From thoseresults, it was confirmed that the silica gel carrying a photocatalystin a high concentration of the present invention hardly decomposes apaint even if the silica gel is mixed with the paint.

Industrial Applicability

The silica gel carrying a titanium oxide catalyst in a highconcentration of the present invention, in which the titanium oxide iscontained with a concentration gradient of titanium oxide in poreshaving a large specific-surface of the silica gel, can adsorb in a largeamount smelly and toxic substances in the air, or organic solvents,agricultural chemicals, etc., contained in water. By irradiating bysunlight, or the light of a fluorescent tube, incandescent lamp, blacklight, ultraviolet lamp, mercury-arc lamp, xenon lamp, tungsten-halogenlamp, metal-halide lamp, or cold cathode fluorescent tube, it is betterthan those in which a titanium oxide film photocatalyst is fixed to onlythe surface of silica gel in its ability and durability to rapidly andefficiently decompose and remove them and to control the propagation offungi, and in its performance that is excellent from the viewpoint ofsafety, economic efficiency, stability, and water-resistance (it doesnot break apart when it is put into water). Further, silica gels invarious forms, such as particulate products, shattered products, andpowdery products, can be used without limitation. Also, the amount oftitanium oxide fixed to the surface of the silica gel can be very small.Thus when it is coated on or contained in fibers, plastics, paints, orpaper, which are organic substances, it poses no problem, and hasfunctions as a substance that decomposes and removes environmentalpollutants. It also has antibacterial and deodorant functions. It can beapplied to a wide range of uses such as those for controlling thepropagation of fungi and for the deodorization of bathrooms, baths thatare available 24 hours, water-purifying devices, air purifiers,lavatories, kitchens, and automobiles, for the treatments of wastewater, cleaning pools and tanks, and preventing the emergence of algaein tanks for aquarium fish.

What is claimed is:
 1. Silica gel carrying a titanium oxidephotocatalyst in a high concentration characterized in that the amountof the titanium oxide contained in the pores near the surface of silicagel is 10-70% by weight, said silica gel having an average pore diameterof 6-100 nm, provided that the concentration gradient is provided suchthat the amount of the titanium oxide contained in the pores near thesurface of the silica gel is 1.5 times or more than the amount oftitanium oxide contained in the pores near the central part of thesilica gel.
 2. A method for preparing silica gel carrying a titaniumoxide photocatalyst in a high concentration, characterized by adding toa sealed vessel silica gel having an average pore diameter in the rangeof 6-100 nm and a titanium-containing solution in the same volume as orless than the total pore volume of the silica gel, and mixing the silicagel and the titanium-containing solution by rotating, vibrating, orshaking the sealed vessel, to have the silica gel contain thetitanium-containing solution, followed by heating and calcining.